Oxygen transport is a crucial process for multicellular animals since every cell requires this gas for oxidation of biomolecules and energy production. However, oxygen solubility in water is insufficient for efficient transfer of the required amount. The carrier based on the iron atoms, coordinated by the protein prosthetic group, became the most effective evolutionary adaptation for solving this problem. Hemoglobin has become one of the most studied molecules of the 20thcentury, being the first protein for which its spatial structure and mechanism of action have been fully described.

Hemoglobin structure

Hemoglobin is a complex protein that is localized in erythrocytes and transports oxygen and carbon dioxide in the blood. There are several forms of this protein; one of these forms, hemoglobin A, which is found in adult humans, is composed of four polypeptide chains, (two α-chains and two β-chains) each with a heme group containing an iron atom [1].

The structure of the protein allows it to easily attach O2 molecules in the conditions with a high concentration of oxygen (in the capillaries of the lung). In addition, each bound O2 molecule makes it easier to attach the remaining molecules. The opposite situation occurs in the tissues, where the pH is more acidic due to the higher concentrations of dissolved carbon dioxide. Under such conditions, hemoglobin, easily releases bound oxygen. Hemoglobin is an allosteric protein. Binding of the first oxygen molecule to one of the protein subunits provokes a conformational change in other subunits that affects the binding of other molecules of the gas and increases the efficiency of conversion of oxyhemoglobin to deoxyhemoglobin.

Red blood cells are almost entirely composed of hemoglobin. Each erythrocyte contains about 280 millions of these protein molecules (96% of dry weight). Given the blood volume of 5 liters, the total weight of hemoglobin in the body is about 800 grams. Blood hemoglobin allows for dissolving 70 times more oxygen than would be possible without this protein [2].

In addition to oxygen and carbon dioxide, hemoglobin binds and transports nitric oxide, carbon monoxide and several other inorganic compounds [3, 4]. The ability of hemoglobin to bind carbon monoxide II (CO) can be very dangerous to humans. The efficiency of this process is 250 times higher than binding to oxygen, therefore carbon monoxide inhalation can quickly lead to toxicity and suffocation.

Hemoglobin is a well-studied protein that was discovered by the German physiologist Otto Funke in 1851, and its structure was first described in 1959 by the Austrian molecular biologist Max Perutz, who later received the Nobel Prize in chemistry for his work [5].